Radiation induced vulcanization of rubber latex



United States Patent 3,131,139 RADTATTSN ENEUCEB VULQANTZATIGN 6h RUBBERLATEX Robert 3. Mesrohim, Finsdale, ill, and Eavid S. Ballantine, BluePoint, and Donald .5. Meta, Stony Brook, N.Y., assignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission No Drawing. (li iginai application Aug. 28, 1959, Ser. No.835,335. Divided and this application Elan. 31, 1963, Ser. No. 263,372

2 Claims. (61. 264-469) This invention relates to new graft copolymers.More particularly, it relates to the preparation of radiationinducedgraft copolyrners by the reaction of a polymerizable ethenicallyunsaturated monomer with polymers of different composition. Moreparticularly, it relates to the technique of radiation grafting ofnatural or synthetic rubber in combination with a vinyl monomer, therebyvarying the properties of these elastomers.

This application is a division of now abandoned application SerialNumber 836,835, filed On August 28, 1959.

To make this invention clear, a brief clarification of terms will behelpful.

By the term graft copolymer, we mean a copolymer, the molecules of whichconsist of two or more polymeric portions, of different composition,chemically united together.

It is lrnown that a quantity of monomer A in bulk or in solution ifexposed to high energy radiation, such as gamma rays, will form a linearpolymer with occasional branching. The product may have the followingform: AAAA-A-AA A A A A A A (1) This is called a homopolymer.

If a mixture of two monomers A and B is exposed to high energy radiationsuch as gamma rays, a polymer is formed which will have monomeric unitscombined in irregulm sequence. Such material, which may be linear orbranched, is called a copoiymer and may have the following generalstructure:

B A A (2) Both copolymers of the type described above have been producedby irradiating monomers and mixtures of monomers of various type in ahigh energy radiation field.

The present invention is directed to the formation of graft copolymersby irradiating with gamma rays a mixture of an ethenically unsaturatedmonomer such as a vinyl monomer and a polymer of different composition.The graft copolymer may have the following generalized form:

Here, the polymeric change derived from monomer B is grafted to variouspoints along the linear chain of polymer A. A similar type of copolymeris the following: AA-A-A-A-B-BBA--A-A-A-A (4) This type is sometimesreferred to as a block copolymer, but we make no distinction herein,referring to both types as graft copolymers. A molecule as (3) or (4)is, in a sense, a copolymer, as the two types of monomer groups arechemically found together in one polymer Patented Apr. 28, 1964molecule. Because of the long sequences of A and B monomeric units, themechanical and solution properties of such grafted copolymers are quitedifferent from those of a copolymer such as (2) where the monomericunits are bound together in irregular sequence. In many respects, agraft polymer resembles a mechanical mixture of polymer A and polymer B,but the components cannot be separated chemically; for example, byextraction using a selective solvent for each of polymer A and polymerB, the graft copolymer is truly a chemical compound.

An object of this invention is the vulcanization of a natural orsynthetic rubber by irradiation.

A further object of this invention i to provide an additive to naturalor synthetic rubber thereby reducing the energetic radiation required tovulcanize natural or synthetic rubber.

Before proceeding to describe our invention further, it may be Well todescribe briefly the effects of high energy radiation such as gammaradiation on polymers and monomers polymerizable by such high energyradiation.

As is now generally understood, monomers are converted into polymers inthe presence of high energy radiation fields through the formation offree radicals. The resulting free radical combines with other monomermolecules in a chain-like fashion to produce a polymeric free radical.The growing polymeric free radical terminates, depending on thepolymerization conditions, to form a polymer A where x indicates thenumber of monomeric units integrated into the formed polymer A Theeffect of high energy radiation, such as gamma rays, on synthetic ornatural organic polymers is likewise regarded as being initiated by freeradicals. Some polyrners, such as polyethylene, natural rubbers andpolystyrene, crosslink in the presence of gamma fields. Other polymers,such as Teflon (polytetrafiuorocthylene) and polymethacrylate degradeit}; a gamma field and form polymers of lower molecular-weighthydrocarbons and hydrogen and other gases.

Vulcanization is the industrial process which transforms rubber from asoft, sticky, readily molded plastic to the highly elastic materialfamiliar to everyone for innumerable uses. It was discovered by CharlesGoodyear in 1839 and involves simply the heating of rubber with sulphur.This process connects the linear polymers by crosslinkages of sulphuratoms. This type of bonding may be illustrated by the following diagram:

o I s l crosslinking, side reactions and such things as cyclization andchain scission which can afiect the quality of the product.

A probable mechanism of gamma ray vulcanization in the elfectuation ofcrosslinlred formations is the breaking is of a CH bond resulting in ahydrogen atom which abstracts another hydrogen atom from a neighboringmolecule, forming a hydrogen molecule. The resultant two free carbonvalances then unite to form a crosslink. It can be anticipated thatvulcanizates containing direct carbon-to-carbon crosslinks will beinherently more stable than sulphur vulcanizates, since the energy ofthe C-to-C bond is 58.6 and that the polysulphide bond only 27.5kilogram-calorie per mole, respectively.

it is lmown that in order to crosslink rubber elfectively, between and50 megarads is needed to achieve the desired tensile strength, modulusand elongation measurement. The addition of the usual chemicalvulcanizing agent, including sulphur, is not seen to have much effect ongamma ray vulcanization. Experiments in which well known free radicalproducing compounds such as organic disulphide, t-butyl hydroperoxide,etc., when added to the rubber produced, at most, only minorimprovements in the gamma ray efiiciency crosslinking. It is theteaching of this invention that in the presence of vinyl monomers suchas 2,5-dichlorostyrene, 3,4-dichloro-1-vinyl cyclohexane and the systemsof the three components, styreneacrylonitrile-divinyl benzene, willincrease the efiiciency of radiation and thereby lower the dosage toabout 1.1 megarads to eflectuate vulcanization.

The following method in which parts and percentages are by weight ispresented in order that the method of preparation as well as the productresulting from the method of our invention may be thoroughly understoodand recognized. This method is not to be taken, however, as limiting theinvention. Controlled experiments were also performed. The controlledones consisted of irradiating the polymer in the absence of a monomer.

A latex, such as Hevea rubber pale crepe, was masticated on a rubber mll to uniform breakdown and manifested a viscosity average molecularweight of 244,000 using the relationship of Carter et al. (Carter, W.C., Scott, R. L. and Magat, M., Natural Rubber, Journal of the AmericanChemical Society, 68, 1480-1485, 1946). Prior to use, the rubber wasshredded and extracted with acetone followed by drying in vacuo. Sheetsof rubber were prepared in thick filmb (ca. 0.2 cm.) by pressure on aheated press or in thin films (ca. 0.005 cm.) by casting from benzenesolution. Monomers, as set forth in Table I, were incorporated into therubber films either by direct contact with liquid or by storage overvapors of monomer. In studies with thin films used for geldetermination, the specimens were degassed and sealed in Pyrex ampules,while the thick films used for physical property measurements weregenerally mounted into an aluminum mold of the type described by Jacksonand Hale (Vulcanization of Rubber with High Intensity Gamma Radiation,Rubber Age, 77, 865-871, 1955). Immediately prior to irradiation thespecimens mounted in aluminum molds were heated for to 60 seconds understeam of 55 p.s.i. gauge pressure at ca. 150 C. The specimens wereexposed to gamma rays of approximately 0.8 mev. and 1.1 megarads perhour intensity from spent fuel elements.

Gel fraction determination of irradiated specimens were performed in twoways. Accurately weighed samples of about 0.4 g. were allowed to standin 100 ml. benzene in the dark for hours. Approximately 0.1 g. ofhydroquinine was added to the benzene solvent to retard oxidation andpolymerization of unreacted monomer. The contents were then filteredunder moderate pressure on a IOO-mesh Nichrome screen and the content ofrubber in sol and gel determined by evaporation to constant weight. Inthe second procedure, which proved to be rapid and more suitable forseveral of the systems, the rubber was placed in chlorobenzenecontaining 0.1% phenyl-betanaphthylamine at 132 C. for several hoursunder a nitrogen atmosphere.

The determination of composition of graft copolymers was performed byelemental analysis.

From the results obtained with 2,5-dichlorostyrene, as revealed by TableIA, it is evident that this compound has a profound effect towardreducing the dosage required to gel natural rubber and that the optimumconcentration of monomer lies in the range of 15 to 21.3 weight percent.It is discernible from Table 13 that in an initial concentration ofabout 15.6 weight percent of dichlorostyrene a dosage of 1.1 megarad issuflicient to gel virtually all network chains, while the non-graftedcontrol requires a dose of 33.4 megarads to produce an equivalent degreeof grafting.

A further effect of graft vulcanization Was found in these inventionsand is illustrated in Table IC. The presence of vinyl polymer serves toreduce the swelling index of vulcanizate below the value of the control.fore, obvious that there exists an infinite variety of monomercombinations and concentrations that can be studied.

Our invention can also be applied to synthetic rubbers such as neopreneand GR-S rubbers.

A fundamental point that arises concerns the mechanism whereby thevarious monomers listed in Tabel I reduce the dose level for gelation(Weight of gel swollen in benzene at 25 C. for 48 hours divided by dryweight of elastomer). One main contribution or" the halogen containingvinyl monomers is their increased radiation sensitizing action whenincorporated in rubber. This effect is indicated in Table IC uponcomparing the gel content of runs made with 2,5-dichlorostyrene and3,4-dichloro-1- vinyl cyclohexane with the control and p-chlorostyrene.However, radiation sensitization by added reagent does not describe theentire effect, since the non-monomeric liquid p-dichlorobenzene producesa result intermediate to the control and the chlorine containingmonomers.

It is postulated that the increase in efiiciency of radiationvulcanization is attributed to several factors which in clude transferof radical sites to alpha-methylenic groups in the rubber as well asinfrequent addition of growing polymer radicals to doubel bonds inrubber.

It is concluded from this invention that the method of simultaneousradiation vulcanization and grafting not only serves to minimize thedose required to effect gelation of network chains but also enables thepreparation of Vulcanizates having a broad range of mechanical andchemical properties.

t is further concluded that a rubber graft copolymer is formed, ratherthan a mixture of vulcanized rubber and polymerized monomer, by the muchlower dose required I to effect gelation when the monomer is present. At6 megarad dosage, only 41.7% of rubber chains are gelled, but in thepresence of monomer; e.g., dichlorostyrene, the entire system (96.1%) isgelled. Since neither the monomer nor the rubber alone could give thishigh gel content it is, therefore, concluded that they have interactedto give a vulcanized graft copolymer.

Since many embodiments may be made of the present invention and sinceany changes might be made in the embodiment described, it is to beunderstood that the foregoing description is to be interpreted asillustrative only and not in a limiting sense.

TABLE I Simultaneous Vulcanizazion of Natural Rubber and GraftCopolymerization With Vinyl [Monomers TABLE IA.EFFEOT OF IMONOMERCONCENTRATION Sec footnotes at end of Table I0.

It is, there- TABLE IB EFFECT OF TOTAL DOSE having a molecular Weight Ofabout and from about to about 21.3 weight percent of 2,5 dichloro-Control 0 29.1 1. 1 16.3

Do 0 1 334 959 styrene to about 1.1 megarads of gamma irradiation Wh11ef f{ f;; 132 iii 3;? 31% 5 maintaining the temperature of the mixture ata tempera- 15.6 56. 4 1.1 97. 6 0 0 15.6 48.9 M 9&0 ture ranging fromabout 56 C. to about 59 C. durlng said exposure.

TABLE IC.EFFECT OF MONOMEB STRUCTURE Wt. percent Temp. Dose Gel con-Tensile Elongation Swelling Added Monomer monomer 0.) (mrad) tent b (wt.stgth. (percent) ratio percent) (p.s.i.)

Control 0 ST. 36.0 97.8 1, 260 540 5 9 D0 0 SJI- 6.0 41.72,5dich1or0styrene 15.6 S.T 6.0 96. 1 1, 490 455 4. 3 Styrene-i-divinylbenzene (96/4 vo1.) l5. 7 S11. 6.0 83. 2 2, 100 190 4. 6Styrene+acrylonitrile (60/40 V01.) 14.1 S.T. 6.0 98.9 2, 240 260 2 7pOhlorostyrene 15.8 S.T 6.0 81. 8 3,4dichloro-1viny1 eyclohexane 14. 3S.T 6.0 93.4 2, 380 180 3 7 Styrene+acrylonitrile+divinyl benzene (58/40v01.) 15.2 S11. 6.0 96.2 3, 740 205 1.8 p-Dichlorobenzene 12.9 S.T 6.068. 7

H Upon repeating several of the earlier runs performed at 1.1 mrad perhour gamma-ray intensity, it was found that heating of samples occursduring irradiation. The values given represent the average temperaturerecorded with thermocouples throughout each run. Where the symbol 8.1.is used, the specimens were held at approximately a source temperatureof 29 C.

b Calculated for total polymer present; i.e., rubber plus polymerizedmonomer.

' Defined as weight of gel swollen benzene at 0. for 48 hours divided bydry weight of elastomer.

References Cited in the file of this patent We claim:

1. A method of vulcaniziug rubber latex comprising UNITED STATES PATENTSexposing a mixture containing latex and from about 15 to 2,956,899 ClineOct. 18, 1960 about 21. 3 Weight precent of 2,5 dichlorostyrene to aboutFOREIGN PATENTS 1. 1 megarads of gamma ray irradiation at a temperature546,817 Belgium Oct 6, 1956 ranging from about 56 C. to about 59 C.during said OTHER REFERENCES exposure.

Lt tl.:Nt l -172Jl11,1953, 2. A method of vulcanizing I-Ievea rubberlatex comg g g a mm W ume u y prising exposing a mixture containingI-Ievea rubber latex BNL 414, October 1956, page 9.

1. A METHOD OF VULCANIZING RUBBER LATEX COMPRISING EXPOSING A MIXTURECONTAINING LATEX AND FROM ABOUT 15 TO ABOUT 21.3 WEIGHT PERCENT OF 2,5DICHLOROSTYRENE TO ABOUT 1.1 MEGARADS OF GAMMA RAY IRRADIATION AT ATEMPERATURE RANGING FROM ABOUT 56*C. TO ABOUT 59*C. DURING SAIDEXPOSURE.